The fuel cell technology is attracting attention as a solution to the problem of energy resources, as well as to the issue of global warming due to CO2 emission. The fuel cell is adapted for electrochemical oxidation of a fuel, such as hydrogen or methanol or any hydrocarbon else in the cell, to effect a direct conversion of chemical energy of the fuel to electrical energy to be taken out. The fuel cell is thus free from emissions of combustion products of fuel, such as NOX and SOX, and attracts attention as a clean energy source for internal combustion engines such as for automobiles, or for thermal power plants.
There are some types of fuel cells, with the PEFC (proton-exchange membrane fuel cell) inclusive, which is now most watched, and developed. The PEFC has various advantages, such that it is (1) adapted for an operation to be facile in start and stop at low temperatures, (2) allowed to be high in theoretical voltage as well as in theoretical efficiency of conversion, (3) implemented with a liquid-free electrolyte allowing a flexible design of cell structure, such as a vertical type, and (4) configured for an interface between ion exchange membrane and electrode to have a three-phase interface controlled to take out an enhanced amount of current, achieving a high density power output.
The principle of operation of a fuel cell includes two electrochemical processes, being an H2 oxidation at the fuel electrode (cathode as negative-pole), and a four-electron reduction of molecular oxygen (O2) shown by formula (A1) below, which produces water.O2+4H++4e−→2H2O  (A1)
This reaction does not have a yield of 100%, and has concurrent side reactions. A typical one of them is a two-electron reducing reaction of oxygen, which produces active oxygen, such as hydrogen peroxide (H2O2), as shown in formula (A2) below, for example. (Kyoto University Graduate School of Engineering as entrustee from the New Energy and Industrial Technology Development Organization, “2001 yearly results report, researches and developments of proton-exchange membrane fuel cell, researches on deterioration factors of proton-exchange membrane fuel cell, fund research (1) on deterioration factors, deterioration factor of electrode catalyst/electrolyte interfaces”, March 2002, p. 27).O2+2H++2e−→H2O2  (A2)
With production of active oxygen due to the two-electron reducing reaction of oxygen, platinum-supporting carbon in the air electrode is oxidized by active oxygen, so that carbon is consumed at the air electrode, where oxygen may have a reduced activation speed on platinum as a catalyst.
The catalyst in use is platinum, which has a lower electrochemical charge voltage than other metals, and a sole metal that can catalyze the electrochemical reaction of fuel cell from about a normal temperature, that is, the reaction of four-electron reduction of oxygen up to water. However, platinum particles may have reduced activities in catalysis, as particle size increases, with exposure to high temperatures as well as high potentials in start-stop operations.
In general, the electrolyte membrane used in PEFC is Nafion® as a cation-exchange film of a perfluorosulfonic acid system. The perfluorosulfonic acid system polymer has a history, where it has been developed as a membrane having a tolerance to active oxygen that the fuel cell produces at the air electrode, i.e., positive electrode. Long endurance tests have not yet revealed a sufficient tolerance. Hydrogen per-oxide produced by the two-electron reduction of oxygen at the air electrode may be the cause. Hydrogen peroxide is stable, and has a long life, though weak in oxidizability. Hydrogen peroxide decomposes, following reaction formulas (A3) and (A4) shown below. When decomposing, it generates radicals, such as hydroxy radical (.OH) and hydroperoxy radical (.OOH). Such radicals, in particular hydroxy radical, are strong in oxidizability, so that even perfluorosulfonated polymer used as an electrolyte membrane may be decomposed in a long use.H2O2→2.OH  (A3)H2O2→.H+.OOH  (A4)
Low-valence ions of transition metals such as Fe2+, Ti3+, or Cu+, if present in the electrolyte membrane, cause a Haber-Weiss reaction, where hydrogen peroxide is one-electron reduced by such a metal ion, producing hydroxy radical. Hydroxy radical, most reactive among free radicals, has a very strong oxidizability, as is known. If the metal ion is an iron ion, the Haber-Weiss reaction is known as a Fenton reaction shown by formula (A5) below.Fe2++H2O2→Fe3++OH−+.OH  (A5)
Metal ions, if mixed in an electrolyte membrane, cause a Haber-Weiss reaction, whereby hydrogen peroxide in the electrolyte membrane is changed into a hydroxy radical, whereby the electrolyte membrane may be deteriorated.
For the electrolyte membrane being acid, PEFC will not work unless the element to be employed as a catalyst is a chemically stable noble metal. Further, for reduction of oxygen at a highest potential in the four-electron reducing reaction, the catalyst to be used there should be an oxidizing agent strong of oxidizability. For such reasons, it is considered difficult to find an alternative to platinum as a catalyst. However, platinum is expensive, and current fuel cell systems require 1 g of platinum per 1 kW. Therefore, assuming an output of an automobile of a class of 2,000 cc in displacement to be 100 kW, conversion for an equivalent fuel cell vehicle (FCV) taken as an example results in 100 g of platinum required per one FCV, which does cost (“Development of Fuel Cell Vehicles & Their Materials” by Kenichiro Ohta, and one else, as supervising editor, CMC Publishers Co., Ltd., December 2002, p. 21). Such being the case, the amount of platinum to be used for elecrodes as well as the development of an alternative catalyst to platinum is important.
As a catalyst employable in place of platinum, there has been proposed, e.g., a catalyst using a cobalt salen compound (Japanese Patent Application Laying-Open Publication No. 2000-251906), or a catalyst using tungsten carbide (Japanese Patent Application Laying-Open Publication No. 2003-117398). Further, to prevent an electrolyte membrane from being oxidized by hydroxy radical, there has been proposed, for example, a method in which a compound with phenolic hydroxyl is mixed in the electrolyte membrane, so that peroxide radicals are Wed to be inactive (Japanese Patent Application Laying-Open Publication No. 2000-223135). There has been proposed another method, in which an electrolyte membrane has a phenol compound, amine compound, sulfur compound, phosphorus compound, or the like mixed therein as anantioxidant to vanish generate radicals (Japanese Patent Application Laying-Open Publication No. 2004-134269). There has been proposed still another method, which has an electrolyte membrane disposed adjacent to a catalyst layer containing molecules having a smaller bond energy than carbon-fluorine bonding in the electrolyte membrane, the molecules reacting with priority to hydroxy radicals, thereby protecting the electrolyte membrane (Japanese Patent Application Laying-Open Publication No. 2003-109623).